Intramolecular vibrational-energy redistribution , unimolecular

Another important question deals with the intramolecular and unimolecular dynamics of the X-—RY and XR -Y- complexes. The interaction between the ion and molecule in these complexes is weak, similar to the intermolecular interactions for van der Waals molecules with hydrogen-bonding interactions like the hydrogen fluoride and water dimers.16 There are only small changes in the structure and vibrational frequencies of the RY and RX molecules when they form the ion-dipole complexes. In the complex, the vibrational frequencies of the intramolecular modes of the molecule are much higher than are the vibrational frequencies of the intermolecular modes, which are formed when the ion and molecule associate. This is illustrated in Table 1, where the vibrational frequencies for CH3C1 and the Cr-CHjCl complex are compared. Because of the disparity between the frequencies for the intermolecular and intramolecular modes, intramolecular vibrational energy redistribution (IVR) between these two types of modes may be slow in the ion-dipole complex.16... 

Vibrational predissociation (VP) of a van der Waals triatomic complex A..BC is an example of a unimolecular reaction the rate of which is controlled by the intramolecular vibrational energy redistribution (TVR) [1]. Within a rigorous quantum mechanical approach, the VP dynamics is completely characterized by the complex-valued energies E = - /T / 2 that lie above the dissociation threshold of A..BC into an atom A and... 

At the unimolecular threshold of moderate to large size molecules (e g. to peptides), there are many vibrational/rotational states within the experimental energy resolution dE and the initial state of A may decay by undergoing transitions to other states and/or decomposing to products. The former is called intramolecular vibrational energy redistribution (FVR) [6]. The probability amplitude versus time of remaining in the initially prepared state is given by... 

If the unimolecular dissociation is not random and not in accord with Eq. (2.2) it is thought that classical bottlenecks restricting intramolecular vibrational-energy redistribution (IVR) may be manifested in the quantum dynamics [16]. Thus, there is considerable interest in identifying the nature of the unimolecular dynamics of excited molecules. In this section Monte... 

For many of the model molecules studied by the trajectory simulations, the decay of P t) was exponential with a decay constant equal to the RRKM rate constant. However, for some models with widely disparate vibrational frequencies and/or masses, decay was either nonexponential or exponential with a decay constant larger than k E) determined from the intercept of P(f). This behavior occurs when some of the molecule s vibrational states are inaccessible or only weakly coupled. Thus, a micro-canonical ensemble is not maintained during the molecule s decomposition. These studies were a harbinger for what is known now regarding inelficient intramolecular vibrational energy redistribution (IVR) in weakly coupled systems such as van der Waals molecules and mode-specific unimolecular dynamics. 

The first step in a unimolecular reaction involves energizing the reactant molecule above its decomposition threshold. An accurate description of the ensuing unimolecular reaction requires an understanding of the state prepared by this energization process. In the first part of this chapter experimental procedures for energizing a reactant molecule are reviewed. This is followed by a description of the vibrational/rotational states prepared for both small and large molecules. For many experimental situations a superposition state is prepared, so that intramolecular vibrational energy redistribution (IVR) may occur (Parmenter, 1982). IVR is first discussed quantum mechanically from both time-dependent and time-independent perspectives. The chapter ends with a discussion of classical trajectory studies of IVR. 

The reason for the observation of the direct trajectory is believed to involve the relation between inefficient stmctural transitions and inefficient intramolecular vibrational energy redistribution. This relation has also been identified in the unimolecular decomposition of fluxional molecules, for... 

RRKM theory assumes the mieroeanonieal ensemble exists at t = 0 and is maintained during the unimolecular decomposition. For the latter to occur, repopulation of the decomposing states results from rapid intramolecular vibrational energy redistribution (IVR). The states are strongly coupled. 

If bottlenecks restrict intramolecular vibrational energy redistribution," the unimolecular dissociation is not random and not in accord with equation (4). There is considerable interest in identifying which unimolecular reactions do not obey equation (4). In this section Monte Carlo sampling schemes are described for exciting A randomly with a micro-canonical ensemble of states and nonrandomly with mode selective excitation. For pedagogical purposes, selecting a microcanonical ensemble for a normal mode Hamiltonian is described first. 

Though not of primary importance to the topic of this chapter, the cyclopropane to propene structural isomerization, it must be noted, has played a prominent role in the development of theory for unimolecular isomerizations and it continues to attract experimental work. The marked dependence of rate on gas pressure and on isotopic substitution exhibited by this isomerizaton provides fundamental insights on intramolecular and vibrational energy transfers and redistributions ... 

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